Lamentable. En Kriptopolis tenéis una descripción del problema, que básicamente consiste en que Microsoft Excel 2007 es incapaz de multiplicar 850 por 77'1 correctamente, ni de gestionar algunas operaciones cuyo resultado sea cercano a 65,535.

excel

Y aquí tenéis la respuesta oficial del fabricante. La culpa, del coma flotante: http://blogs.msdn.com/excel/archive/2007/09/25/calculation-issue-update.aspx

So what, specifically, are the values that cause this display problem? Of the 9.214*10^18 different floating point numbers (floating point on wikipedia) that Excel 2007 can store, there are 6 floating point numbers (using binary representation) between 65534.99999999995 and 65535, and 6 between 65535.99999999995 and 65536 that cause this problem. You can’t actually enter these numbers into Excel directly (since Excel will round to 15 digits on entry), but any calculation returning one of those results will display this issue if the results of the calculation are displayed in a cell. All other calculation results are not affected.

No os perdáis los comentarios de los usuarios. Entre tanto, os recuerdo que existen alternativas a Excel que nada tienen que envidiarle:

IBM Lotus Symphony Spreadsheets
OpenOffice.org Calc